In a wireless digital communication system, a station obtains access to the medium for transmission (e.g., a radio frequency) by deploying an access algorithm. A system according to the IEEE 802.11 Wireless LAN standards, for example, typically uses the Carrier Sense Multiple Access (CSMA) algorithm. The CSMA algorithm initially listens to the medium and begins transmitting when the signal level is below a certain threshold. Even with the CSMA algorithm, however, collisions will occur. For example, a collision will occur when two or more stations start transmitting at the same moment. The 802.11 standard provides a mechanism to reduce this probability by using a random back-off counter and using priority windows for transmitting certain packets. Even when a collision does not occur, a frame may not be correctly transmitted or acknowledged.
The 802.11 standard contains an acknowledgement mechanism to ensure that messages are properly received. After receiving a message in good order, the receiving station sends out an acknowledgement (ACK) message to the transmitting station. This ACK mechanism protects the system against packet loss, e.g., due to collisions. If a collision occurs, one or two stations will not receive an ACK message on their transmitted message. Nonetheless, an ACK message may not be received on a transmitted message, for example, due to (i) an inherent detection failure (a false alarm probability versus a detection probability); (ii) bad signal quality (too much time dispersion); (iii) low signal power (too large a distance); or (iv) interference at the receiving station.
A given 802.11 implementation has several mechanisms available to overcome the inability to successfully transmit a message. If the transmitting station did not receive an ACK message, the transmitting station may retransmit the original message up to N times. The transmitting station increases the random access time with each retransmission. During any of these retries, the transmitting station may do retransmissions using any suitable modulation. For example the transmitting station may successively retransmit the original message on a lower rate (thereby providing more robust modulation), according to a rate fall-back algorithm. This proceeds until the maximum number of retries, N, is reached or until the ACK message is received. The data transport layer at the destination station can conclude that a frame was lost from, for example, a gap in the sequence numbers that the transport layer assigns to each packet. A lost frame can have a negative impact on the performance of the total system. For example, the TCP/IP transport layer will typically reduce its send rate when several lost packets are detected, so that the total throughput of the system is temporarily reduced.
A need therefore exists for an improved rate fallback method and apparatus for retransmitting a lost packet. A further need exists for a mechanism to increase the probability that a frame is correctly transmitted and acknowledged, so that the above-described negative impact on the data transport layer will not occur.